1. Field of the Invention
The present invention relates to a method of forming a metal line in a semiconductor device. More specifically, the present invention relates to a method of forming a metal line of a semiconductor device, wherein line resistance of the line can be reduced, and line burial characteristics and electrical characteristics can be improved.
2. Discussion of Related Art
As the line width of semiconductor devices was miniaturized and the degree of integration thereof increased, it became impossible to fulfill a desired line width through a line formation method using an existing Reactive Ion Etch (RIE) process. Recently, lines have been formed by means of a damascene method.
In case of the line formation method using the damascene method, a metal line material has to be deposited/buried in a pattern region that is formed in the form of a trench, unlike the RIE method.
A source contact of a NAND flash memory device is a type of this trench pattern, and it has a three-dimensional structure unlike the contact structure. It is thus difficult to deposit and bury the metal line material.
Generally, in semiconductor devices, tungsten (W) using a Chemical Vapor Deposition (hereinafter, referred to as “CVD”) method is used as a main metal line material. Before tungsten (W) is deposited, TiSix for electrical ohmic contact and a glue layer for improving adhesion are formed. The glue layer generally employs a Ti film.
When CVD tungsten (W) is the main metal line material, WF6 is used as a deposition gas. Fluorine (F) of WF6 is infiltrated from the bottom of a trench into a semiconductor substrate, thus increasing line resistance. In a worse case, a short problem (hereinafter, referred to as “F-attack”) is generated. There also occurs a problem in that tungsten (W) of WF6 and the Ti film being the glue layer react to each other in an explosive manner (hereinafter, referred to as “W volcano”). It is thus required to form a barrier metal layer having a predetermined thickness using a material such as TiN, TaN or TiW, which has a resistance relatively higher than that of the main metal line material. In the case of flash memory devices of 70 nm grade, a barrier metal film is formed to a thickness of about 100 Å or more.
Recently, a TiN film having good step coverage, which is deposited by means of CVD method, has generally been used as the barrier metal film. The CVD TiN film is formed to a constant thickness regardless of the top, middle and bottom portions of a trench due to its good step coverage. The TiN film formed on the sidewalls of the trench causes a volume of a portion where a subsequent main metal line material will be buried to be reduced. The inlet of the trench becomes narrow toward the top of the trench, thus causing a seam or void. This seam or void further reduces the volume of the main metal line material, and thus results in an increased line resistance.
More particularly, the metal line of the damascene structure is a three-dimensional structure unlike RIE method, and is thus largely dependent upon the structure of the glue layer and the barrier metal film. Furthermore, this phenomenon becomes more profound when the line structure is miniaturized, and thus causes failure in deposition and burial of the main metal line material. It results in reduction of the volume of the line and formation of the void within the line, thus increasing resistance. It also has a bad influence on electrical characteristics.
The easiest and most reliable method for improving the burial characteristics and electrical characteristics of the main metal line material is to reduce a thickness of the barrier metal film and thus to increase the importance occupied by the main metal line material. If the thickness of the barrier metal film is lower than a predetermined thickness, however, there is a problem in which resistance of a semiconductor substrate increases due to WF6 gas since the original object of the barrier metal film is lost, or a F attack problem in which the line and the semiconductor substrate are shortened, a W volcano problem in which the glue layer (Ti) under the barrier metal film and WF6 react to each other in an explosive way, and the like.